Oklahoma plans to start using nitrogen gas for executions, officials said on Wednesday, in what would be the first such method of capital punishment in the United States.

The state is turning to nitrogen after it and other states became unable to acquire drugs required for lethal injections because of opposition from manufacturers to their products being used for capital punishment.

Nitrogen is an odorless and tasteless gas that makes up about 78 per cent of the air humans breathe but causes death when inhaled without any oxygen.

Oklahoma has not carried out an execution since 2015 after a series of mishaps, including a botched lethal injection where an inmate was seen writhing in pain and another inmate who was executed using a drug not approved by the state.

Oklahoma Attorney General Mike Hunter said in a statement that the state would use nitrogen hypoxia – asphyxiation by breathing in the inert gas – as its primary means of execution once a protocol had been finalized for the process.

“Using an inert gas will be effective, simple to administer, easy to obtain and requires no complex medical procedures,” Hunter said in the statement.

Under state law, if lethal injection is unavailable, executions must be carried out by nitrogen gas inhalation, the statement added. A spokeswoman for Hunter declined to comment on when executions might resume.

Robert Dunham, executive director of the Death Penalty Information Center, a capital punishment monitor, said it would be some time before the state could seek a death warrant for an execution using nitrogen.

He said it faced a court order requiring it to wait 150 days until after a use protocol was published and would likely face litigation.

Dunham was not aware nitrogen hypoxia had ever been used anywhere in the world to execute a person. He said the American Veterinary Medical Association deemed the process inappropriate for euthanizing mammals and said it would take more than seven minutes to bring about the death of a 70-pound (32-kg) pig.

“This is another execution process that is untested, untried and experimental” Dunham said.

Hunter’s statement cited unnamed studies that said nitrogen hypoxia would lead to death in “just a few minutes.”

Dale Baich, one of the attorneys for the 20 Oklahoma death row inmates challenging the state’s method of execution, said the state needed to show what scientific research it had to prove the safety and legality of this new process.

“Without complete transparency, we have no assurance that executions won’t continue to be problematic,” Baich said in a statement.